Polycrystalline Ni Films Research Articles

Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition

In this work we present a low cost and scalable technique, via ambient pressure chemical vapor deposition (CVD) on polycrystalline Ni films, to fabricate large area ( approximately cm2) films of single- to few-layer graphene and to transfer the films to nonspecific substrates. These films consist of regions of 1 to approximately 12 graphene layers. Single- or bilayer regions can be up to 20 mum in lateral size. The films are continuous over the entire area and can be patterned lithographically or by prepatterning the underlying Ni film. The transparency, conductivity, and ambipolar transfer characteristics of the films suggest their potential as another materials candidate for electronics and opto-electronic applications.

Crystallite size dependence of the coefficient of thermal expansion of metals

The coefficients of thermal expansion (CTEs) of polycrystalline Ni and Cu thin films have been investigated by employing temperature-dependent x-ray diffraction measurements of lattice parameters. Great care has been taken to exclude effects of, in particular, microstructural relaxation and mechanical stresses on the dependences of the lattice parameters on temperature. The CTEs determined in the as-deposited condition, characterized by grain sizes in the range of 25–35nm, are considerably (about 10%) larger than the corresponding literature values of bulk materials. Heat treating the specimens at moderate temperatures induced grain growth and decrease of the crystalline imperfection. After the heat treatment, the CTEs determined for the thin films had reduced considerably and had become equal to (Ni) or approached (Cu) the corresponding literature data for bulk materials.

Ion-induced magnetic texturing of Ni films: Domain structure and strain

Ion-irradiation of ferromagnetic films induces pronounced changes of their microstructure and micromagnetism. The present study is devoted to the changes in the domain structure in 65–75nm electron-evaporated polycrystalline Ni films due to 200-keV Xe+ or 100-keV Ni+ implantation. For magnetic analysis, we combined magnetic force microscopy (MFM) and magneto-optical Kerr effect (MOKE); X-ray diffraction and Rutherford back-scattering spectroscopy served to characterize the microstructure of the films. After deposition, MFM indicated a pattern of magnetic ripples (with in-plane and perpendicular components of the magnetization). After ion irradiation, MOKE showed almost total in-plane remanence, and MFM confirmed the disappearance of the ripples at room temperature. The magnetic properties were correlated with the ion-induced changes in the strain in the as-deposited and ion-irradiated films.

Ion-beam induced changes in magnetic and microstructural properties of thin iron films

Changes in magnetic and structural properties of 60–82 nm iron films induced by heavy-ion implantation were studied using the magneto-optical Kerr effect, Mossbauer spectroscopy, Rutherford backscattering spectroscopy, X-ray diffraction, and X-ray absorption fine structure. The influence of ion-beam parameters (ion mass, fluence) and of sample parameters (external magnetic field and stress during implantation) were investigated. The Fe films, some of them containing a thin 57Fe marker layer for Mossbauer spectroscopy, were deposited on Si(100) substrates, by electron-beam and effusion-cell evaporation. The films were irradiated with 20Ne, 56Fe, 86Kr and 132Xe ions at energies chosen so that the implantation profiles peaked near the middle of the Fe films. The as-deposited films were magnetically isotropic and had a high coercivity. After ion implantation, the coercivity decreased and magnetic anisotropy developed. Both changes correlated with a decrease in the internal film stress. External mechanical stress applied during the irradiation had hardly any influence on the magnetic texture, opposite to an external magnetic field applied during or before ion implantation. The results are compared with those obtained for ion-irradiated polycrystalline Ni films and epitaxial Fe films and discussed with respect to the role of radiation-induced extended defects as pinning centers.

Pulsed-laser deposition of polycrystalline Ni films: A three-dimensional kinetic Monte Carlo simulation

A three-dimensional (3D) kinetic Monte Carlo simulation was performed for the growth of polycrystalline Ni films on a Ni(1 0 0) substrate via pulsed-laser deposition (PLD) on the basis of the deposition of the incident atoms that nucleate randomly and the adatoms surface diffusion combining with a growth restriction within identically oriented grains. The simulations showed that the grains grow rapidly in PLD until they reach the critical grain size that is about 7 monolayer (ML) for the substrate temperature of T = 300 K, and then grow slowly with the time span of the simulations. Moreover, we compared PLD with molecular beam epitaxy (MBE), and found that PLD is characteristically different from MBE at the beginning of the deposition under the condition of the same deposition rate in both cases. However, the differences became not obvious as time unfolds and growth proceeds later.

Reorientation phase transition and sensitivity of the extraordinary Hall-effect-based sensors

Spin reorientation phase transition was observed in 5–10 nm thick polycrystalline Ni films at about 150 K. The extraordinary Hall effect resistivity in these films is enhanced by surface scattering and is of the order of 1 μΩ cm. Magnetization reversal in the vicinity of the transition is very sharp due to the development of the multi-domain structure with the out-of-plane anisotropy. As a result, the field sensitivity of the Hall resistance reaches values exceeding 500 Ω/T.

Magnetic texturing of xenon-ion irradiated nickel films

Thin polycrystalline Ni films of typically 75 nm thickness evaporated on Si or SiO2 substrates were irradiated with 30-900 keV Xe-ions to fluences of 2.5 x 1013 - 4 x 1014/cm2. The magnetization of the Ni films was measured using the longitudinal Magneto-Optical Kerr Effect and Vibrating Sample Magnetometry. The Ni-film thickness and Xe-concentration profiles were determined with Rutherford backscattering spectroscopy and the lattice dilation with X-ray diffraction. The Xe-irradiations were found to induce an in-plane uniaxial magnetic anisotropy within the Ni-films. This magnetic texture was investigated in relationship to the microstructure as function of the ion energy and fluence, the sample temperature, the presence of an external magnetic field during the irradiation and the stress field produced before, during and after the implantations.

Characterization of magnetic tunnel junctions (MTJ) with magnetostrictive free layer materials

Abstract Magnetic tunneling junctions (MTJs) incorporating magnetostrictive free layer materials are developed and characterized for the application as highly sensitive strain sensors. Gauge factors of the order of 400–600 have already been demonstrated compared to typical values of 2–4 for metal film based strain sensors. Here, different magnetostrictive materials such as amorphous FeCoSiB alloy and polycrystalline Ni films have been investigated in order to characterize and optimize the MTJ based sensor properties.

Large spin polarization in epitaxial and polycrystalline Ni films

The spin polarization $(P)$ of tunneling electrons from epitaxial and polycrystalline Ni films was investigated using Meservey-Tedrow technique. The tunneling electrons from the Ni(111) surface were predominantly of majority spin, giving a $P$ of $+25%.$ This is the first observation of the $P$ of tunneling electrons from Ni(111) in a planar tunnel junction. A large increase in $P$ was observed for a polycrystalline Ni film $(P=+46%)$ when the Ni surface/interface was cleaner. Our results point to the important role of localized minority $d$ states near ${E}_{F}$ in defining a higher majority $P$ of the itinerant electrons seen in transport measurements, further showing the feasibility to tailor new materials with even larger $P$ values.

Effect of GaAs substrate on the magnetic properties of Ni film

In this paper, temperature and field dependent magnetization processes of Ni films are studied. Polycrystalline Ni films are deposited on n-type GaAs (0 0 1) and Si (0 0 1) substrates by both the usual thermal evaporation and the UHV deposition in a MBE system. The roughness and the inherent oxide layer of the substrate are taken into consideration while preparing the Ni film on GaAs substrate. For a comparative study, similar Ni films deposited on glass substrate and a bulk Ni sample are investigated as well. At low temperature, considerably large applied fields are required to gradually saturate the Ni/GaAs films, starting from the end of the hysteresis. As an associate phenomenon, a large reduction of the remanent moment is observed. Coercive field as high as 71 mT is observed for this Ni film on GaAs substrate at 4.2 K. These anomalous magnetic behaviors are considered to be the effects of GaAs substrate, as they do not appear in Ni films deposited on Si substrate, glass substrate, and in the bulk Ni sample. A local antiferromagnetic interaction among the Ni spins at the interface of Ni and GaAs has been speculated. This interaction becomes stronger as the temperature goes below the room temperature.

Temperature dependence and annealing effects on magnetoelastic SAW attenuation and magnetoresistance in Ni films

The magnetic field dependencies of 600 MHz magnetoelastic Rayleigh-mode surface acoustic wave (SAW) attenuation and dc magnetoresistance of Ni thin films were measured as a function of temperature and correlated with variations of the samples' anisotropy field. Thin film samples ( 10 dB/cm) in SAW attenuation response. This effect may be due to irreversible magnetoelastic annealing effects between the thin film and substrate. After annealing, the subsequent temperature dependence of sample response repeatably correlated with a linear (-0.6 Oe/K) temperature dependence of the samples' uniaxial anisotropy field. The temperature dependent anisotropy may be caused by the combined effects of the magnetostriction and differential thermal expansion between the polycrystalline Ni films and the anisotropic quartz substrates. These effects can be used to provide a method for enhancement of the sample magnetoelastic SAW attenuation and magnetoresistive response by controlling the magnitude and direction of the sample's anisotropy field.

KLL Auger transitions in metallic Cu and Ni

The X-ray excited KLL Auger spectra of polycrystalline Cu and Ni films, measured with high energy resolution and presented here, show intensive satellite structures of intrinsic origin. A good agreement is between the determined relative KL 23 L 23 energies or KL 3 L 3 relative intensities and their experimental (Cu) and theoretical (free atoms) values obtained earlier. However, the deviation is significant for the absolute Auger transition energies and very large for the relative intensity of the Ni KL 2 L 2 ( 1 S 0 ) line or of the initial state shakeup satellites.

Ion-beam-induced magnetic texturing of thin nickel films

The texturing of thin metallic and ceramic films caused by heavy-ion implantation is a common observation, but still lacks quantitative explanations. Magnetic texturing effects of polycrystalline Fe-films irradiated with Xe ions have recently been reported. In this paper, we present results of similar experiments for 18–400 nm thick polycrystalline Ni-films on Si (1 0 0) substrates after irradiating them with 2×10 14–1×10 16 Xe +-ions/cm 2 at 77 K and at ion energies of 6–450 keV, in the presence of a small magnetic field of the order of 1 G. The magnetization was measured by means of the magneto-optical Kerr effect (MOKE), as function of several parameters such as fluence, energy and spot size of the ion beam, thickness and shape of the Ni film and the thermal annealing procedures after irradiation.

The electrophysical properties of thin polycrystalline Cr, Cu, Ni and Ti films

The electrophysical properties of thin polycrystalline Cr, Cu, Ni and Ti films

Microstructure and magnetoelastic coupling coefficients in thin NiFe and Ni films

The effective magnetoelastic (ME) coupling coefficients Beff of polycrystalline Ni79Fe21 and Ni films have been reported as functions of thickness t. Beff was observed to have a thickness-independent bulk term, Bb, and a thickness-dependent surface term, Bs/t consistent with the Néel model. For thickness below 50 Å the surface term is important; it can change the sign of the effective ME coupling constant and dramatically increase its magnitude. We expect that the surface ME coupling we observe in polycrystalline thin films may be a combination of the intrinsic Néel surface effect and extrinsic microstructure effects. Our transmission electron microscopy, Auger depth profiling, and atomic force microscopy rule out film discontinuity and compositional gradients as significant sources of the surface ME effect.

Surface magnetoelastic coupling coefficients of single-crystal fcc Co thin films.

We propose a method of analysis that allows indirect determination of magnetoelastic coupling coefficients for thin magnetic films. The method is based on a general phenomenological equation describing the dependence of the effective magnetic anisotropy energy density on film thickness for a thin cubic ferromagnetic film sandwiched between two nonmagnetic layers of the same material. We show that by fitting the measured effective anisotropy energy density with this equation using independently measured elastic strain in the film and the saturation magnetization ${\mathit{M}}_{\mathit{s}}$, we can extract the N\'eel surface magnetic anisotropy energy density ${\mathit{K}}^{\mathit{s}}$ and either of the surface magnetoelastic coupling coefficients ${\mathit{B}}_{1}^{\mathit{s}}$ or ${\mathit{B}}_{2}^{\mathit{s}}$ depending on the film orientation. The model is applied to published data for fcc Co/Cu(111) superlattices. We find that ${\mathit{B}}_{2}^{\mathit{s}}$=-23.5 erg/${\mathrm{cm}}^{2}$ and ${\mathit{K}}^{\mathit{s}}$(Co/Cu)(111)=+0.47 erg/${\mathrm{cm}}^{2}$. While such large values of ${\mathit{B}}^{\mathit{s}}$ relative to ${\mathit{K}}^{\mathit{s}}$ are surprising, recent direct measurements of ${\mathit{B}}^{\mathit{s}}$ in polycrystalline Ni films show these values to be reasonable.

Giant surface magnetostriction in polycrystalline Ni and NiFe films

We have measured the effective magnetoelastic coupling coefficients, Beff, of polycrystalline NiFe/Ag/Si, NiFe/Cu/Si, and Ni/SiO2/Si films in situ as functions of magnetic layer thickness over the range from 1.5 to 50 nm using magneto-optic Kerr effect and applied static strain. The Beff’s agree well with bulk values at large thicknesses but take on anomalously large values for thicknesses below about 5 nm. The data are well fit by a Néel model, Beff=Bbulk+Bsurf/(t−t0), where t0 may be related to intermixing at the interface with the substrate (verified by Auger depth profiling). These data suggest that the surface conditions are of enhanced importance in controlling magnetic properties, particularly anisotropy, permeability, and noise, even in films whose compositions are nominally of zero magnetostriction.

In-plane magnetic anisotropies in polycrystalline Ni films induced by Xe bombardment during growth

250 to 1500 AA thin Ni films were ion beam sputtered onto a fused quartz substrate with simultaneous bombardment by Xe ions of 100 eV. Hysteresis loops were recorded ex situ by the longitudinal magnetooptic Kerr effect. A maximum in-plane uniaxial anisotropy field of 12 kA/m (151 Oe) was found in 750-AA-thick films that were deposited with a flux ratio of Xe ions to Ni atoms of 0.12. The easy axis of magnetization was found to lie perpendicular to the plane of incidence of the secondary ions. Films showing the strong uniaxial in-plane anisotropy were almost completely

The measurement of magnetostriction constants of thin films using planar microwave devices and ferromagnetic resonance

In this paper we introduce a new technique for measuring the saturation magnetostriction constant (λs) for isotropic polycrystalline thin films. The technique makes use of nonresonant planar microwave structures together with a novel stressing mechanism to induce a shift in the resonant field of a magnetic thin film as measured by a ferromagnetic resonance (FMR) experiment. Measurement of the shift induced by a uniaxial stress allows for determination of λs via a magnetic resonance analysis. Either a slotline device or coplanar waveguide (CPW) was used as the source of the microwave excitation field depending upon the orientation of the dc magnetic field. To evaluate the technique, polycrystalline Ni films of thickness of 637 nm were sputtered onto glass substrates at room temperature for comparison with the literature. Using a 50-Ω CPW, FMR measurements at 9 GHz revealed an average value of λs of −36×10−6 a value in agreement with those previously reported. The technique provides increased flexibility over other FRM techniques as it is wideband and directly accessible.

On the nature of ion bombardment-induced phase transformations in films of transition metals

Abstract Electron diffraction studies have been made of polycrystalline Ni films irradiated with well separated beams of ions of different nature, namely ions of inert (He+, Ne+, Ar+, Kr+, Xe+) and reactive (N+ and O+) gases. The Ni films were prepared under vacuum conditions (P≳ 3·10−6Pa during evaporation) preventing an appreciable contamination of the films with impurities. The samples were irradiated at T≍ 300 K with ion beams of energies from 10 to 100 keV in the dose range between 5·1016 cm−2 and the value leading to sample destruction. Irradiation with noble gas ions revealed no phase transitions in the Ni films. A similar result was obtained in irradiation of Fe and Cr films with He+ ions. The bombardment of Ni films with reactive gas ions does cause changes in the lattice structure of the samples under study, depending on the nature of the bombarding ions. The N+ ion bombardment gives rise to the hcp phase with the lattice parameters typical of the Ni3N compound, and the O+ ion bombardment result...